The present invention relates to a video signal copy protection process and apparatus, and more particularly to a technique for providing or enhancing copy protection of video signals via the addition of special negative-going pulses as well as special positive-going pulses in some embodiments, within selected portions of front porch regions in the video signal horizontal blanking interval (HBI).
In the field of video signal copy protection, copy protection processes are well known for modifying a video signal so that a normal color picture may be produced by a television receiver receiving the copy protected video signal, but wherein videotape recording of the copy protected video signal is inhibited and/or is so degraded in quality as to be unwatchable. In addition, many processes have been developed which enhance or otherwise improve upon the many basic copy protection processes.
Typical of such processes is that disclosed in U.S. Pat. No. 4,631,603 issued to J. Ryan in Dec. 23, 1986, and herein incorporated by reference, which capitalizes on the fact that a typical video cassette recorder's (VCR) automatic gain control (AGC) system cannot distinguish between the normal sync pulses of a conventional video signal (including equalizing or broad pulses) and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to a normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video signal during the vertical blanking interval, and each of such pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback. However, this approach utilizes positive-going pulses which occur at a time not ordinarily used for black-level restoration in typical television receivers and therefore does not give rise to black level depression in such typical television sets, whereby a television set (TV) displays a picture of fully acceptable quality.
Another copy protection process is disclosed in U.S. Pat. No. 4,819,098 issued to J. Ryan on Apr. 4, 1989 and incorporated herein by reference, which capitalizes on the fact that VCRs include the AGC system which here measures the sync pulse level in a video signal and develops a gain correction signal for keeping the video level applied to an FM modulator in the VCR system at a fixed, predetermined value. A plurality of positive pulses are added to the video signal with each immediately following a respective trailing edge of a normally occurring sync pulse. These added pulses are clustered at the vertical blanking interval of each field to minimize the affect of the same on the watchability of the picture defined by the signal, but still cause the automatic level control circuit in a VCR to assess the video level at many times its actual value thereby providing a copy of unacceptable picture quality. The sync pulses themselves can also be at a reduced level, in order to enhance the effectiveness of the copy protection process.
A further copy protection process is disclosed in U.S. Pat. No. 4,163,253 issued to M. Morio et al. on Jul. 31, 1979, wherein the AGC circuit of a VCR again prevents recording an acceptable copy of a copy protected video signal. A pulse whose amplitude is substantially equal to the peak white level of the video signal is inserted into respective horizontal blanking intervals of the video signal. If this modified video signal is recorded on a conventional type of video signal recorder, the presence of the inserted pulse signal is detected by the AGC detector which, in turn, controls an AGC amplifier to substantially attenuate the modified video signal which then is recorded. The resulting recording is of unacceptable picture quality.
It has been found that the copy protection signals of various copy protection processes which employ positive-going pulses, such as AGC pulses, in the back porch region of the HBI, may experience various degrees of playability problems such as for example black level depression. In the field of copy protection, it obviously is necessary to maximize the effectiveness of the copy protection of a recorded video signal. However, it is equally necessary to maintain the playability of the copy protected video signal by reducing the amount of side effects such as darkening of the TV set monitor during playback of the video signal, playback of a legal copy of the video signal, etc. It follows that it would be advantageous to maximize not only the effectiveness of a copy protection process against illegal copying but also the playability of the video signal by a legitimate TV set.